When improving upon an existing design, take the time to thoroughly inspect and gather as much information as possible about the broken part.

Looking closer at the broken OEM end cap, you can see an external random texture. Such random textures are common on high-content glass reinforced plastic parts such as those used in power drills.

On close inspection, the fracture line reveals that the part is not actually glass reinforced. The texture is also absent on the inside walls of the part and there are no plastic designations or stamps on the inside walls.

The circles on the inside of the part also reveal that the part was indeed injection molded.